Computational Intelligence

Neural Network History 161

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Figure 5.2 A simplified four-PE Hopfield network.

Hopfield used values of 1 and 0, but subsequently showed that values of 1 and –1

result in simplified mathematics. We use 1 and –1. The value that the PE assumes is

governed by a hard-limiting function. By this we mean that if the net input to a PE

is greater than or equal to some threshold value (usually defined to be 0), then the

activation value is 1; otherwise, it is –1.

Before we review the operation of the network, two limitations of Hopfield net-

works should be mentioned. The first is that they can reliably store and recall only

about 15